Assessment of Eco-friendly Gases for Electrical Insulation to

Subscriber access provided by READING UNIV

Critical Review

An Assessment of Eco-friendly Gases for Electrical Insulation to Replace the Most Potent Industrial Greenhouse Gas SF6 Mohamed Rabie, and Christian Michael Franck

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.


Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03465 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017



Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts



appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor



changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.


Page 1 of 48

Environmental Science & Technology

Fi r s t p ate nt

Fi r s t G I S

St a r t o f N OA A / E S R L

E PA i n U S

M a r k e t e nt r y

of a synthetic gas (CFC-12) for electrical insulation

installed with SF6 as insulation and quenching gas

global atmospheric measurements showing an increase of SF6 abundance

“SF6 Emission Reduction Partnership for Electric Power Systems”

of synthetic insulation and quenching gases other than SF6

1889

1967

1937 Fi r s t i nve s t i g at i o n s of dielectrics of synthetic gases, e.g. R-10, R-20,...

1997

1987

1938

1995

2006 1999

2015

Fi r s t p ate nt s

M o nt re a l Pro to co l

Kyo to Pro to co l

E U R e g u l at i o n 8 4 2

of SF6 as insulation and quenching medium in electrical equipment

phase-out of ozonedepleting substances (CFCs, HCFCs)

SF6 listed as one of the six greenhouse gases

on reporting, monitoring, recycling and disposal of SF6

ACS Paragon Plus Environment

(a)


Page 2 of 48

8

SF6 (ppt)

6

5

4 2

(b)

AGAGE ( ) NOAA ( ) CDIAC

0

∆ T (m°C)

6 4 2

0 1960


1980 year

2000

2020

SF6 emissions (Gg/yr)

Page 3 of 48 (a)

8 6

2

(b)

6

A global/top−down B global/electric/EDGARv4.2 C global/total/EDGARv4.2 D Annex I/total/UNFCCC E Annex I/electric/UNFCCC

4

0 SF emissions (Gg/yr)


3 2

1970

1980

1990

2000

2010

2005

2010

A EU/total/UNFCCC B EU/electric/UNFCCC C US/total/UNFCCC D US/electric/UNFCCC E China/total/EDGARv4.2 F China/total/bottom−up G China/total/top−down

1 0


1990

1995

2000


15

Switzerland

Page 4 of 48 total (top-down) total (reported) Electrical (reported)

SF6 emissions (Mg/yr)

10 5 0

80

United Kingdom

60 40 20 0 1990

Plus Environment 2005 1995ACS Paragon2000 year

2010

p (MPa)

Page 5 of 48 (a)


10

0

10

−1

10 10

−2

−3

O2 CO2 SF 6 CF3 I C 4F 7N C 5 F 10 0 C 6 F 12 0

(b) p (MPa)

N2

2 0 −200

HFO 1234ze(Z) HFO 1234ze(E) HFO 1234yf

ACS Paragon Plus Environment −100 T (°C)

0

100


Page 6 of 48

leakage Sulfur hexafluoride

long-lived greenhouse gas Electrical Switchgear


Page 7 of 48


An Assessment of Eco-friendly Gases for Electrical Insulation to Replace the Most Potent Industrial Greenhouse Gas SF6 1

Mohamed Rabie∗ and Christian M. Franck Power Systems and High Voltage Laboratories, ETH Zurich, 8092 Zurich E-mail: [email protected]

2

Abstract

3

Gases for electrical insulation are essential for the operation of electric power equip-

4

ment. This review gives a brief history of gaseous insulation that involved the emergence

5

of the most potent industrial greenhouse gas known today, namely sulfur hexafluoride.

6

SF6 opened up the way to space-saving equipment for the transmission and distribution

7

of electrical energy. Its ever-rising usage in the electrical grid also played a decisive role

8

in the continuous increase of atmospheric SF6 abundance over the last decades. This

9

Review broadly covers the environmental concerns related to SF6 emissions and assesses

10

the last generation of eco-friendly replacement gases. They offer great potential to re-

11

duce greenhouse gas emissions from electrical equipment but at the same time involve

12

technical trade-offs. The rumours of one or the other being superior seem premature,

13

in particular due to the lack of dielectric, environmental and chemical information for

14

these relatively novel compounds and their dissociation products during operation.

1



15

1

Introduction

16

Today’s transmission and distribution of electric power in densely populated areas is signifi-

17

cantly and increasingly dependant upon space-saving electric power components. Equipment

18

manufacturers address the challenges of both the electric and thermal stress onto the com-

19

pact components mainly with sophisticated equipment design and special materials including

20

solid, liquid, gaseous and vacuum insulation. 1 In principle, all types of electrical insulation

21

media can achieve elevated voltage and current ratings of equipment, along with compound-

22

specific trade-offs such as the equipment weight, outer dimensions, durability, flexibility,

23

safety, maintenance effort, production costs, or environmental issues. The choice of the

24

actual insulation medium is additionally influenced by historical developments, political in-

25

struments and social factors.

26

The applications of gaseous insulation - often containing components other than air and

27

above atmospheric pressure - aim to avoid discharges in compact apparatuses. 2–10 The use

28

of gases as electrical insulator in medium and high voltage equipment has many advantages

29

over liquid and solid insulation such as relatively low weight, low costs, simple manufacturing

30

process of equipment, full recovery of insulation performance after partial discharge and the

31

ability of insulating moving parts. In transformers, bushings and gas insulated transmission

32

lines (GIL) the insulation gas solely serves as an electrical insulator, whereas in high voltage

33

circuit breakers of air-insulated switchgear or substations (AIS) and gas insulated switchgear

34

(GIS) the gas must comply with both electrical insulation and - if no vacuum interrupters are

35

used - current interruption. GIS can be installed in densely populated areas and therefore

36

support network topologies with lower power losses. As shown in several life cycle assessment

37

studies, this can potentially lead to an overall reduced CO2 footprint in comparison to

38

AIS. 11,12

39

In particular for high voltage applications, SF6 has been the electrical industry’s favored

40

insulation and arc quenching medium for half a century. 13,14 For very cold regions, SF6 must

41

be mixed with a more volatile carrier gas such as N2 or tetrafluoromethane CF4 to avoid 2


Page 8 of 48

Page 9 of 48


42

liquefaction. 15 GILs, typically filled with 20% of SF6 in nitrogen, can be an alternative to

43

cables, if the installation of overhead transmission lines is challenging due to space limita-

44

tions or lack of public acceptance. 7,16,17 In radar systems of military reconnaissance planes,

45

including the Airborne Warning And Control System (AWACS), SF6 operates as an electrical

46

insulating medium in the hollow conductors of the antenna. 18 In particle accelerators, e.g. in

47

the waveguide of linear accelerators for medical radiotherapy, SF6 is used to prevent sparking

48

and provide cooling for the dielectric load. 8 In Resistive Place Chambers (RPC), which are

49

e.g. used as particle detectors in all large experiments of the Large Hadron Collider (LHC), 9

50

SF6 is used very diluted (≤ 1%) due to its desired quenching effect on streamer discharges. 10

51

After being classified as one of the greenhouse gases in the Kyoto protocol, regulative

52

measures for the SF6 usage have been implemented in various industries. The electrical

53

industry as the largest contributor to reported SF6 emissions 19–24 has been excluded from any

54

SF6 ban so far. In the short term, emissions have been reduced by voluntary commitments

55

of the electrical industry by minimizing the use and leakage rates from electrical equipment,

56

improving gas handling and recycling concepts. 25,26 In the long term, gradually reducing the

57

quantities of greenhouse gases that can be placed on the market has been identified for other

58

industries as the most effective way of reducing emissions. 27 For a phase-out of SF6 from

59

electrical power equipment, a replacement gas with acceptable environmental impact would

60

be beneficial.

61

2

62

In the very early beginnings of gaseous dielectrics, the focus has been on compounds other

63

than SF6 . In pioneering experiments that he reported to the Royal academy of Sciences of the

64

Austro-Hungarian Empire in 1889, K. Natterer first noticed the high electric strength of some

65

substances, such as CCl4 , CHCl3 or SiCl4 , relative to that of air or nitrogen. 28 He applied

66

high-voltage pulses generated by an induction coil to the vapor of more than 50 different

Brief history of gaseous insulation

3



Page 10 of 48

67

substances. There is no evidence that Natterer’s discoveries aimed on technological advances

68

by potentially using gases of high electric strength in electrical equipment. Only decades

69

later, the usefulness of these compounds as electrical insulation media were recognized in

70

the United States. In the early 1930s, R.G. Herb noticed that the maximum obtainable

71

voltage of a pressurized-air insulated Van de Graaff generator is significantly increased by

72

using CCl4 . 29 Moreover, he found together with M.T. Rodine that when adding CCl4 to air

73

the electric strength rises more rapidly in the region of low CCl4 -concentration than at high

74

concentrations. This effect, known as synergism, is generally a quality criteria also for the last generation of insulation gases.

Fi r s t p ate nt

of a synthetic gas (CFC-12) for electrical insulation

1889

1937

Fi r s t i nve s t i g at i o n s of dielectrics of synthetic gases, e.g. R-10, R-20,...

Fi r s t G I S

1938

Fi r s t p ate nt s

St a r t o f N OA A / E S R L

installed with SF6 as insulation and quenching gas

1987

1967

of SF6 as insulation and quenching medium in electrical equipment

“SF6 Emission Reduction Partnership for Electric Power Systems”

1997

1995

M a r k e t e nt r y

E PA i n U S

global atmospheric measurements showing an increase of SF6 abundance

of synthetic insulation and quenching gases other than SF6

2006

1999

2015

M o nt re a l Pro to co l

Kyo to Pro to co l

E U R e g u l at i o n 8 4 2

phase-out of ozonedepleting substances (CFCs, HCFCs)

SF6 listed as one of the six greenhouse gases

on reporting, monitoring, recycling and disposal of SF6

Figure 1: Timeline regarding substances used for electrical insulation and crucial events concerning the use and the development of insulation gases. 75

76

In the same years, E. E. Charlton and F. S. Cooper found that halocarbons generally have

77

higher electric strength than nitrogen, 2 and in particular proposed CCl2 F2 , also known as

78

the refrigerant CFC-12, which is non-toxic, non-corrosive and chemically stable, for the use

79

in high voltage transformers. 3 Fifty years later the phase-out of CFC-12 was decided within

80

the Montreal Protocol due its role in ozone depletion. Furthermore, E. E. Charlton and F.

81

S. Cooper investigated tetrafluoromethane CF4 , which is used until today in admixture with

82

SF6 in polar regions. 4


Page 11 of 48


83

Besides highly chlorinated compounds, K. Natterer run into another gas of high electric

84

strength containing the cyano (-CN) functional group, namely cyanogen (C2 N2 ). The electric

85

strength of this toxic inorganic compound, was determined with more accurate experimental

86

methods to be around 2 times the one of nitrogen. 30 With some detours over straight-chain

87

nitriles, such as CF3 CN, C2 F5 CN or C3 F7 CN, 6,31 that have toxicities higher than would

88

be considered acceptable for insulation purposes, the (-CN) group recently reappeared in

89

the branched nitrile (CF3 )2 CFCN (or C4 F7 N), which is a last generation insulation gas of

90

"acceptable toxicity". 32

91

Shortly after being patented as gaseous fire extinguishing substance, 33 SF6 was for the

92

first time patented as an insulation medium by F. S. Cooper from the American producer

93

General Electric 34 and as an arc quenching media by V. Grosse from the German producer

94

AEG in 1938. 35 In his patent, V. Grosse already explained that its exceptional ability of

95

quenching arcs is due to its high heat capacity and its dissociation during arcing and the

96

subsequent recombination of the dissociation products.

97

In the 1930-40s dielectric properties of various potential insulation gases including SF6

98

were experimentally determined. 2,3,29,36–39 It has been speculated upon the reasons for their

99

relatively high electric strength that crucial factors are the large molecular weight, com-

100

plexity and electron affinity, since they affect the interaction between free electrons and gas

101

molecules. More elementary studies by F. M. Penning pointed out that the development of

102

an electron avalanche is suppressed in gases of small first Townsend coefficient (nowadays

103

known as effective ionization coefficient), which quantifies the electron multiplication in an

104

electron avalanche by electron impact. 40 The picture of an electron avalanche as the source

105

of the electric breakdown led H. Raether, J. M. Meek and L. B. Loeb to identify the first

106

Townsend coefficient as the principal controlling factor in the breakdown voltage of a gap in

107

given geometrical conditions and particularly at high pressures. 41–43 Small Townsend coef-

108

ficients should consequently result in relatively high breakdown voltages. Exactly this was

109

experimentally observed in SF6 and other halogenated gases by B. Hochberg and E. Sand-

5



110

berg. 39 They further speculated that the small Townsend coefficient for halogenated gases

111

resulted from the inefficient production of new electrons by impact ionization or from the

112

higher electron energy losses in inelastic non-ionizing collisions with these rather complex

113

molecules in comparison to oxygen or nitrogen. However, other complex molecules such

114

as hydrocarbons did not show such a high electric strength. It was not until the 1950s,

115

supported by mass spectrometer studies, 44,45 that the halogenated molecules’ ability to ef-

116

fectively attach free electrons has become generally recognized to be responsible for the

117

small first Townsend coefficient and the high electric strength of halogenated gases. 46,47 It

118

was found that negative ions are created by the two-body process of dissociative attachment

119

such as in CCl4 , 47 SF6 47,48 and Trifluoroiodomethane CF3 I, 30 or by the three-body process

120

of electron attachment where a dissociation process does not occur. 48,49

121

From all halogenated substances, SF6 turned out to be the best insulation gas in terms of

122

stability, toxicity and liquefaction temperature, although other gases revealed higher electric

123

strengths. The industrial production of SF6 began in the 1950s with the first commercial

124

SF6 circuit breakers in the United States. 20 With the market introduction of GIS in the late

125

1960s in Europe, companies began the move away from oil towards SF6 , and the use of SF6

126

as an arc quenching and insulation medium became widespread. On the transmission level,

127

AIS were more and more replaced by GIS. Also on the distribution level AIS have been

128

gradually replaced by GIS, the latter being more compact, reliable and safe in operation.

129

In the late 1970s/early 1980s, a search for alternative gases superior to SF6 was initiated

130

by Westinghouse Electric Corporation, 4 General Electric 6 and Oak Ridge National Labo-

131

ratory. 5 The goal of the systematic investigation was to find a gas or gas mixture that has

132

lower production costs, lower boiling point, and an electric strength that is higher and less

133

sensitive to surface roughness and particles. Ozone-depleting substances such as CFC-12 as

134

well as strong greenhouse gases such as perfluorocarbons (PFCs) 50 or CF3 SF5 with a GWP

135

(100-year) of 17400 24 were considered. Even though compounds were not selected with re-

136

spect to their environmental impact, no gas that would result in technical and economic

6


Page 12 of 48

Page 13 of 48

137


advantages to the electrical industry could be identified.

138

In the late 1980s, environmental concerns related to anthropogenic emissions of halo-

139

genated compounds arose for the first time with the discovery of large losses of total ozone

140

in the Antarctic ozone layer. 51 This led to the formation of the Montreal Protocol which

141

resulted in the phase-out of chlorofluorocarbons (CFCs) and later hydrochlorofluorocarbons

142

(HCFCs), which were used at the time as refrigerants, blowing agents and aerosols. The

143

healing of the Antarctic ozone layer due to emission reductions of CFCs has meanwhile been

144

proven. 52 The electrical industry was not affected by political measures or bans since SF6 is

145

not ozone-depleting. However, the classification of SF6 as a greenhouse gas by the United

146

Nations Framework Convention on Climate Change in 1992 and later by the Kyoto Protocol

147

affected also the use of SF6 . As a consequence, policy makers and environmental agencies

148

increasingly expressed concern about the climate impact of this extremely strong greenhouse

149

gas. This initiated new attempts to replace SF6 by alternatives with lower global warming

150

potential. 53,54 In 1999, the United states environmental protection agency (EPA) initiated

151

the "SF6 Emission Reduction Partnership for Electric Power Systems" based on a voluntary

152

partnership with the members of the U.S. electric power industry 26 and in 2009 classified SF6

153

as "a threat to the health and welfare of current and future generations due to their effects

154

on world climate" under section 202(a) of the Clean Air Act. In 2006 the EU Regulation No.

155

842/2006 55 was put into force by the European Parliament and the Council, replaced in 2014

156

by the EU Regulation No. 517/2014. 27 This regulation makes, among other things, provision

157

to the European commission for an assessment of SF6 replacements in new medium voltage

158

secondary switchgear no later than 2020 and for other applications, including high voltage

159

equipment, no later than 2022. In 2012, the Australian government applied an equivalent

160

carbon tax on synthetic greenhouse gases including SF6 , which was repealed in 2014. 56

161

In parallel to the increasing number of regulative measures, the resumption of research

162

activities in SF6 replacements resulted in the development of equipment filled with the at-

163

mospheric gases CO2 , N2 and O2 57,58 and mixtures of these gases with synthetic compounds

7



164

of significantly lower GWP than SF6 . 59–62 The overall performance of some of these alterna-

165

tives come very close to the one of SF6 , although some trade-offs such as larger equipment

166

size, higher filling pressures, slightly thicker walls of vessels or higher minimum operating

167

temperatures are necessary. 63

168

3

169

Anthropogenic emissions of extremely long-lived and potent greenhouse gases might irre-

170

versibly change the climate on millennium timescales. 24,64 SF6 has a very pronounced ab-

171

sorption band exactly at an infrared frequency where the earth’s atmosphere is relatively

172

transparent and therefore absorbs upward radiance 42000 times more effectively than CO2

173

(compare radiative efficiencies in table 2). The IPCC currently reports an atmospheric life-

174

time of 3200 years and a GWP value of 23500 (100 years time horizon), 24 assuming photolysis

175

of SF6 by UV radiation as the main removal process. However, the most recent estimate of

176

the atmospheric lifetime of SF6 is 850 years (uncertainty range from 850-1400 years), with

177

electron attachment being identified as the main removal process. 65 This reduced lifetime

178

yields a GWP value of ∼ 22500.

179

3.1

180

Pre-industrial atmospheric SF6 abundance has been less than 6.4 ppq (parts per quadrillion)

181

as known from measurements of air trapped in Antarctic firn. 66 Today, SF6 abundance is

182

three orders of magnitude higher. This is known with high precision due to sampling pro-

183

grams that measure background atmospheric concentrations of SF6 and other trace species

184

using gas chromatographs, as shown in figure 2(a). The NOAA/ESRL program starting

185

1995 67 and the AGAGE program starting 2001 68 show that SF6 concentrations globally

186

increase and are nearly equal over the northern and southern hemisphere.

187

Climate impact of SF6

SF6 abundance and climate response

The equilibrium global mean surface temperature response ∆T = λ · RF due to the

8


Page 14 of 48

Page 15 of 48


188

radiative forcing of SF6 (RF = RE · [SF6 ]) is shown in 2(b), with the equilibrium climate

189

sensitivity parameter being λ = (1.5 ◦ C − 4.5 ◦ C)/3.7 Wm−2 . 24 The transient temperature

190

rise might be - in comparison to the equilibrium temperature response - time-delayed by the

191

heat capacity of the earth climate system and the timescale for heat transport into the deep

192

ocean. 24,69 The current equilibrium warming due to SF6 is 0.004 ◦ C, with a clear tendency

193

to increase.

194

Emissions scenarios by 2100 have been given in the Fifth Assessment Report of the IPCC

195

(table AII 4.4 24 ). Table 1 summarizes the reported projections to 2030, 2050 and 2100 for

196

SF6 emissions and abundance 24 as well as the corresponding ∆T . The worst case emissions

197

scenario in this report (A2) assumes a continuously increasing population and regionally

198

oriented economic development, and would yield ∆T ∼ 0.03 ◦ C due to SF6 in 2100. In

199

view of the Paris Agreement within the UNFCCC of keeping the increase in global average

200

temperature to well below 2 ◦ C, under the A2 scenario a SF6 emission cut today could

201

contribute 1.5% of this goal. Table 1: SF6 emissions, abundance and equilibrium temperature rise due to SF6 radiative forcing for the best (RCP2.6) and worst (A2) emissions scenario. year

emissions (Gg/yr) 2030 2-12 2050 1-16 2100

Assessment of Eco-friendly Gases for Electrical Insulation to

Recommend Documents